Time-resolved absorption spectroscopy on the femtosecond time scale has been used to monitor the earliest events associated with excited-state relaxation in tris-(2,2'-bipyridine)ruthenium(II). The data reveal dynamics associated with the temporal evolution of the Franck-Condon state to the lowest energy excited state of this molecule. The process is essentially complete in approximately 300 femtoseconds after the initial excitation. This result is discussed with regard to reformulating long-held notions about excited-state relaxation, as well as its implication for the importance of non-equilibrium excited-state processes in understanding and designing molecular-based electron transfer, artificial photosynthetic, and photovoltaic assemblies in which compounds of this class are currently playing a key role.

The synthesis and photophysical characterization of a series of aryl-substituted 2,2‘-bipyridyl complexes of RuII are reported. The static and time-resolved emission properties of [Ru(dpb)3](PF6)2, where dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been examined and are contrasted with those of [Ru(dmb)3](PF6)2 (dmb = 4,4‘-dimethyl-2,2‘-bipyridine). It is shown through analysis of electrochemical data and detailed fitting of the emission spectrum that the unusually large radiative quantum yield for [Ru(dpb)3](PF6)2 in CH3CN solution at room temperature is due to reduction of the degree of geometric distortion along primarily ring-stretch acceptor mode coordinates relative to other molecules in this class. It is proposed that the 3MLCT excited state of [Ru(dpb)3]2+ is characterized by a ligand conformation in which the 4,4‘-phenyl substituents are coplanar with the bipyridyl fragment, leading to extended intraligand electron delocalization and a smaller average change in the C−C bond length upon formation of the excited state as compared to [Ru(dmb)3]2+. These conclusions are further supported by photophysical data on several new molecules, [Ru(dptb)3](PF6)2 (dptb = 4,4‘-di-p-tolyl-2,2‘-bipyridine), [Ru(dotb)3](PF6)2 (dotb = 4,4‘-di-o-tolyl-2,2‘-bipyridine), and [Ru(dmesb)3](PF6)2 (dmesb = 4,4‘-dimesityl-2,2‘-bipyridine). The systematic increase in steric bulk provided by this ligand series results in clear trends in k r, k nr, and S M (the Huang−Rhys factor), consistent with the delocalization model. In addition, time-resolved resonance Raman data reveal frequency shifts in ring-stretch modes across the series supporting the notion that, as the steric bulk of the ligand increases, the ability for the peripheral phenyl rings to become coplanar with the bipyridyl fragment is hindered. Ab initio calculations employing Hartree−Fock and second-order perturbation theory on neutral and anionic 4-phenylpyridine, put forth as a model for the ground and excited states of [Ru(dpb)3]2+, are also reported. These calculations suggest a canted geometry for the ground state, but a considerable thermodynamic driving force for achieving planarity upon reduction of the ligand. The canted ground-state geometry is also observed in the single-crystal X-ray structure of the mixed-ligand complex [Ru(dmb)2(dpb)](PF6)2. Finally, consideration of how this system evolves from the Franck−Condon state to the planar thermalized 3MLCT state is discussed with regard to the possibility of time-resolving the onset of extended electron delocalization in the excited state by using ultrafast spectroscopy.

For the first time, new catalysts for olefin polymerization have been discovered through the application of fully integrated high-throughput primary and secondary screening techniques supported by rapid polymer characterization methods. Microscale 1-octene primary screening polymerization experiments combining arrays of ligands with reactive metal complexes M(CH(2)Ph)(4) (M = Zr, Hf) and multiple activation conditions represent a new high-throughput technique for discovering novel group (IV) polymerization catalysts. The primary screening methods described here have been validated using a commercially relevant polyolefin catalyst, and implemented rapidly to discover the new amide-ether based hafnium catalyst [eta(2)-(N,O)[bond](2-MeO[bond]C(6)H(4))(2,4,6-Me(3)C(6)H(2))N]Hf(CH(2)Ph)(3) (1), which is capable of polymerizing 1-octene to high conversion. The molecular structure of 1 has been determined by X-ray diffraction. Larger scale secondary screening experiments performed on a focused 96-member amine-ether library demonstrated the versatile high temperature ethylene-1-octene copolymerization capabilities of this catalyst class, and led to significant performance improvements over the initial primary screening discovery. Conventional one gallon batch reactor copolymerizations performed using selected amide-ether hafnium compounds confirmed the performance features of this new catalyst class, serving to fully validate the experimental results from the high-throughput approaches described herein.

Crystal structures of bis (methyl[8] [bis ([8]annulenyl)ynerbium(lnl (14) and uranocene (1). A comparison of the structural parameters of several additional [8]annulenyl metal complexes is also presented. The effect of coordination number on metal ligand distances in ionic complexes is discussed in terms of electrostatic interactions among the ligands. IntroductionOne of the most important factors affecting the structure and properties of an organometallic compound is the oxidation state of the central metal. In order to assess the effect of a change in oxidation state on the structural characteristics of a compound it is necessary to compare compounds in which there is little or no change in the coordinating ligands. However, the number of systems of this type for which structural information is available is quite limited, the principal examples thus far being the metallocenes. For example, in ferrocenel the metal-carbon distance is shorter than in ferrocenium cation2 despite the fact that the ionic radius of six-coordinate Fe(lll) is generally smaller than that of six-coordinate Fe(II). A rationalization based on a covalent model suggests that oxidation of ferrocene removes an electron from a metal-ligand bonding orbital, resulting in an increase in the metalcarbon distance.We have studied the effect of a change in oxidation state of the central metal on the structural parameters of bis ([8] Compound numbers are those given in Table 1. Results and Discussion Note on the comparison of structurally related compoundsWe compare structures ofbis [8]annulene complexes differing both in oxidation state of the central metal and in substitution of the [8]annulene rings, with the implicit assumption that substitution on the rings does not significantly affect the parameters we will be comparing, e.g. average metal-ligand bond distances. As evidence of this assumption, note that the structures of 2 and 3 are essentially the same as that of unsubstituted uranocene (1) with respect to metalring carbon distance and ring planarity. In the structures of all complexes containing Comparison of the structures of bis(methyl[8]annulenyl)cerium(IV) (5) and [(diethylene glycol dimethyl ether)potassium][bis([8]annulenyl)cerate(III)] (13)Greco26 et al. have reported preliminary x-ray structure data that suggest that Ce(CsHsh (cerocene) is isostructural with (CgHg)2U (uranocene, 1) and (CgHg)2Th 3 (thorocene, 4); however, a fully solved structure has not been published. Repeated attempts at our hands to obtain crystals of unsubstituted cerocene for a single crystal x-ray analysis have proven fruitless, but suitable crystals of 1,1'-dimethylcerocene (5) were isolated. The ORTEP of the molecular structure is presented in Figure 1 and selected distances and angles are summarized in Tables 4 and 5 U(lm (176°) (vide infra) and U(IV).lO Solid state and gas phase structures29 of several alkaline-earth and lanthanide bis(pentamethylcycloi>entadienyl) metallocenes exhibit non-linear ring-centroid-metal-ring-centroid angles; angles in these co...

The discovery of new olefin polymerization catalysts is currently a time-intensive trial-and-error process with no guarantee of success. A fully integrated high-throughput screening workflow for the discovery of new catalysts for polyolefin production has been implemented at Symyx Technologies. The workflow includes the design of the metal-ligand libraries using custom-made computer software, automated delivery of metal precursors and ligands into the reactors using a liquid-handling robot, and a rapid primary screen that serves to assess the potential of each metalligand-activator combination as an olefin polymerization catalyst. "Hits" from the primary screen are subjected to secondary screens using a 48-cell parallel polymerization reactor. Individual polymerization reactions are monitored in real time under conditions that provide meaningful information about the performance capabilities of each catalyst. Rapid polymer characterization techniques support the primary and secondary screens. We have discovered many new and interesting catalyst classes using this technology.